The fluidic stability of a burner affects the occurrence of thermo-acoustic oscillations. Fluidic instability waves occurring at the burner can result in the formation of vortices or “coherent structures” that can influence the combustion process and lead to periodic releases of heat with the associated fluctuations in pressure. These high-amplitude fluctuations in pressure can result in a limitation of the operating range and can increase the emissions associated with the combustion. These problems occur particularly in combustion systems with low acoustic damping as often represented by modern gas turbines. Particularly in the lean combustion range there can be a periodic loss of flame stability that also results in pulsations.
Coherent structures play a crucial role in mixing processes between air and fuel. The spatial and temporal dynamism of these structures influences the combustion and the release of heat. A process is known from EP 0 918 152 A1 in which means for acoustic excitation of the working gas were arranged in the vicinity of the burner to counter the occurrence of coherent structures. This process provided for the shear layer formed in the area of the burner to be excited in order to require as little excitation energy as possible. The momentary acoustic excitation of the shear layer was mode locked with a signal measured in the combustion system in order to determine the excitation energy to be input and its frequency. This process requires, however, extensive means for controlling the thermo-acoustic oscillations.
A method is known from DE 100 56 124 A1 in which the flame position is influenced by means of a graduated injection of the fuel and hence the influence of fluidic instabilities and also time delay effects is reduced. For this, pickups to measure the pulsations and emissions of the combustion and regulating devices to control the graduated injections are used.
The adaption of the mixing profile in the burner can also have a direct influence on the pulsations and emissions. DE 100 64 893 A1 discloses a burner with a graduated injection in which the fuel outlet orifices are divided into at least three groups and the fuel mass flow of the groups can be axially symmetrically controlled independently of one another via valves. Opposed nozzles are thereby grouped together and not controlled independently of one another.
The essentially random variation of the mixing profile allows flame form and flame position to be changed. This enables the influence of fluidic instabilities and also time delay effects to be reduced. The occurrence of fluctuations in the heat release and hence the thermo-acoustic oscillation are reduced as a result.